ISSN:0006-3525    

DOI:10.1002/bip.1968.360061002

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1968

数据来源: WILEY

摘要:

AbstractA method is reported for calculating the melting curve of a DNA molecule of random base sequence, including in the formalism the dependence of the free energy of base pair formation on the size of a denatured section. Some explicit results are shown for a “typical” base sequence, in particular the probability of helix formation at individual base pairs in several different regions of the molecule and the amount of melting from the end of the chain. Particular attention is drawn to the variation of local melting behavior from one region of the molecule to another. It is found that sections rich in AT melt at relatively low temperatures with a fairly broad transition curve, whereas regions rich in GC pairs melt at higher temperatures (as expected) with a very abrupt, local transition curve. To account qualitatively for the results one may divide melting into two kinds of processes: (a) the nucleation and growth of denatured regions, and (b) the merging together of two denatured sections at the expense of the intervening helix. The first of these processes dominates in the first stages of melting, and leads to rather broad local melting curves, whereas the second process predominates in the later stages, and occurs, in a particular part of the molecule, over a very narrow temperature range. It is estimated that the average length of a helix plus adjacent coil section at the midpoint of the transition is approximately 600 base pairs. Since transition curves which measure the local melting behavior reflect local compositions fluctuations, these curves contain information about the broad outlines of base sequence in the molecule. Some suggestions are made concerning experiments by which this potential information source could be exploited. In particular, it is pointed out that one might hope to map AT or GC rich regions at particular genetic loci in a biologically active DNA molecule. Values of the relevant parameters found earlier for the transition of homopolymers produce melting curves for a DNA of random base sequence which are in good agreement with the experimental transition curve for T2 phage DNA. Hence the present theoretical picture of the melting of polynucleotides is at least internally self‐consi

ISSN:0006-3525    

DOI:10.1002/bip.1968.360061003

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1968

数据来源: WILEY

摘要:

AbstractCopolymers of vinylimidazole and vinylpyrrolidone containing 0.7–24% of the former residue have been prepared. Titration behavior was examined over a range of temperature and at several ionic strengths. Intrinsic pK's, corrected for electrostatic effects, arc independent of mole fraction of imidazole residue in the copolymer. Nevertheless they are about 1.15 pKunits lower than in the corresponding model small molecule,N‐ethylimidazole. Enthalpies and entropies of ionization in the copolymer are changed by ionic strength. It is apparent that the charged state of anN‐ethylimidazole residue placed in a macromolecular matrix is destabilized relative to the uncharged state because of changes in the local polymer or solvent enviro

ISSN:0006-3525    

DOI:10.1002/bip.1968.360061004

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1968

数据来源: WILEY

摘要:

AbstractCopolymers of vinylimidazoles and vinylpyrrolidinone, dilute in imidazole residues, have been prepared; their reactivity in the hydrolysis ofp‐nitrophenyl acetate has been examined. The catalytic activity of imidazole residues in these copolymers is comparable to that of nonpolyineric imidazoles of the same basicity, and no kinetic effects of preassociation of polymer with substrate ester ofp‐nitrophenol were detected. It is suggested that although polyvinylpyrrolidinone probably does bind these substances, they are sequestered from the catalytic, side cha

ISSN:0006-3525    

DOI:10.1002/bip.1968.360061005

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1968

数据来源: WILEY

摘要:

AbstractThe general conformations of a system of three linked peptide units are studied, and it is found that there are three types of conformations which contain NH…O hydrogen bonding between the first and the third units. One of them is part of a 310‐helix, while the other two arc nonhelical. The two nonhelical conformations are very similar, and in both the cases the peptide chain turns around, reversing the direction of progress. Such a conformation can therefore occur in the region where a polypeptide chain folds back on itself, as in the cross‐β structure. The method of representing these interesting tripeptide conformations in a (ϕ,ψ) map is described. Examples of such hydrogen‐bonded, nonhelical conformations which occur in peptides and proteins are discussed—e.g., in cyclohexaglyeyl, an open tetrapeptide Gly‐L‐Pro‐L‐Leu‐Gly, and in parts

ISSN:0006-3525    

DOI:10.1002/bip.1968.360061006

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1968

数据来源: WILEY

摘要:

AbstractBirefringence of poly‐A, poly‐U, and double‐stranded poly‐AU are investigated experimentally as well as theoretically. Single‐stranded poly‐A, at low ionic strength, exhibits a strong negative birefringence indicating that, adenine molecules are stacked regularly with their planes along the transverse direction. On the addition of Mg or Mn ions, the birefringence decreases markedly. Poly‐U has no noticeable birefringence indicating that poly‐U has a contracted configuration in solution. On mixing poly‐A and poly‐U with 10–3MMgCl2, the birefringence increases markedly, but the change in the chain configuration seems less pronounced. The polarizability of adenine and uracyl are ejaculated using the simple Ifïückel scheme. The calculation shows that the π‐electron polarizability normal to the base plane is zero. Therefore, the negative birefringence is probably due to the lack of π‐electron polarizability in the direction normal to the base plane and accordingly no π‐electron polarizability along the major helix axis. Conversely, the negative birefringence i

ISSN:0006-3525    

DOI:10.1002/bip.1968.360061007

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1968

数据来源: WILEY

摘要:

AbstractAt low frequencies, the dielectric constant of gels of agar, carboxymethyl cellulose, gelatin, mid maize starch is much higher than that of water. It decreases continuously as the frequency increases, tending to level off at about 108cps. The dielectric constant is lower the higher the concentration of polymer; it increases in the order: carboxymethyl cellulose, agar, starch, and gelatin. Results are most reasonably explained by assuming a more solid structure of water the lower the dielectric constant.

ISSN:0006-3525    

DOI:10.1002/bip.1968.360061008

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1968

数据来源: WILEY

摘要:

AbstractThe adsorption at the air‐water interface of calf thymus H3‐DNA labeled in the dry state by the Wilzbach method was studied by measuring surface concentration, surface tension, and surface potential. It was found that, correspondingly to the behavior ofE. coliH3‐DNA labeled by incorporation of thymine H3and described in another paper, both the rate of adsorption and the amount of material adsorbed increased with increase in DNA concentration, salt concentration, or in the valency of the counterion. Surface pressure and potential did not change in the course of adsorption, and this is also in accordance with the properties found forE. coliDNA. However, while the surface concentration of theE. coliDNA corresponds approximately to monomolecular layer adsorption, the radiation from the adsorbed layer of the calf thymus H3‐DNA indicated apparently multilayer adsorption. On comparing the physical properties of the H3‐DNA labeled by the Wilzbach method to those of nonlabeled DNA, it is found that while the chemical composition and the bihelical structure is essentially maintained in the labeled material, exposure to tritium gas results in a reduction in molecular weight and produces random breaks in the strands of the bihelix. The H3‐DNA produced by Wilzbach's method is not labeled homogeneously. The more the molecule is exposed to the gaseous tritium, the more efficient is the isotopic, exchange and the greater the alteration in physical properties. The defects in the labeled H3‐DNA molecule make it more surface active, thus H3‐DNA of higher specific radioactivity concentrates at the interface, conveying the impression of multilayer formation although actually the adsorbed layer is approximatel

ISSN:0006-3525    

DOI:10.1002/bip.1968.360061009

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1968

数据来源: WILEY

摘要:

AbstractCircular dichroism (CD) measurements were carried out on various copolymers ofL‐tryptophan and γ‐ethylL‐glutamate in ethylene glycol monomethyl ether as the solvent. On increasing theL‐tryptophan content of the copolymers a gradual change in the CD spectra was observed. The typical spectrum of the right‐handedα‐helix becomes more and more evident as theL‐tryptophan content decreases. On the basis of these results we assumed that no conformational transition occurs on proceeding from pure poly (γ‐ethylL‐glutamate) to pure poly‐L‐tryptophan in ethylene glycol monomethyl ether: therefore the conformation of poly‐L‐tryptophan should be that of a right‐handed α‐helix. Moreover we observed that the change in the CD spectra of the copolymers is gradual but not linear on increasing the tryptophan content. The deviations from linearity were attributed to interactions among side‐chain chromophores whose contributions to the optical activity are not simply additive. An x‐ray analysis carried out on oriented films of poly‐L‐tryptophan casted from solutions of the polymer in dimethylformamide shows conclusively that the solid‐state conform

ISSN:0006-3525    

DOI:10.1002/bip.1968.360061010

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1968

数据来源: WILEY

ISSN:0006-3525    

DOI:10.1002/bip.1968.360061011

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1968

数据来源: WILEY